Infrared (IR) radiation detectors provide an electrical output which is a measure of incident IR radiation. One particularly useful IR detector is a photovoltaic (PV) detector fabricated from Group II-VI radiation absorbing semiconductor material, such as mercury-cadmium-telluride (HgCdTe). HgCdTe detectors are typically fabricated as linear and two-dimensional arrays. Generally, a transparent substrate supports a radiation absorbing semiconductor layer having a first electrical conductivity, and a second semiconductor layer of opposite electrical conductivity forms a p-n junction with the first layer. The array may be differentiated into a plurality of p-n junctions by selectively removing the semiconductor material, resulting in the formation of a plurality of upstanding "mesa" structures, each of which contains a radiation detecting element, or pixel. The array typically includes a layer of passivation applied to an outer surface so as to reduce surface states and resulting noise signals that detrimentally affect the operation of the p-n junction. An anti-reflection (AR) coating may also be applied over the passivation layer to reduce reflections of incident radiation. Each of the radiation detecting elements includes a contact, normally provided in the form of one or more square or round metal pads. The contact pad(s) provides electrical contact for external read-out circuitry, typically via an indium "bump" interconnect, to the p-n junction. If radiation enters the array through the bottom surface of the substrate, that is, the surface opposite to the surface that supports the radiation absorbing semiconductor layer, the array is referred to as a "backside-illuminated" array.
Incident radiation may be directed to the array with a beam steering device such as a dual prism, microprisms, or an array of diffractive binary lens elements. It is conventional practice to tilt or rotate the detector array through the use of a beam streering device about one axis such that a radiation receiving surface of the array is inclined at an angle to incident radiation.
A problem that is presented during the use of such detector arrays results from reflection of radiation from array features, such as the edges of the contact pads, mesa sidewall surfaces, and the like. This reflected radiation is radiation that first passes unabsorbed through the substrate, the radiation absorbing semiconductor layer, and the overlying layer of opposite conductivity. This unabsorbed radiation eventually encounters the array "top-side" edges and features and is reflected therefrom back through the body of the array. If the reflected radiation is not absorbed during the second pass through the array, the radiation emerges from the bottom surface of the substrate and may propagate back into space. This propagating radiation signal is often referred to as a "light signature". Significant contributors to the light signature are reflections from the contact pad edges and mesa edges.
As was stated, in prior detective assemblies the focal plane array was tilted, or tipped, in only one axis. However, this arrangement leaves many array features (edges of mesas and contact pads) orthogonally disposed to the illuminating beam, resulting in relatively large optical signatures from these edges.
It is thus an object of this invention to significantly reduce the light signature of a focal plane array.
It is a further object of this invention to significantly reduce the optical signature of a focal plane array by simultaneously tipping the focal plane features (optically and/or mechanically) in both major array axes, without degrading the imaging performance of the detective assembly.